This invention relates to a process for binding pigment particles to phosphor particles using polymers or copolymers applied as latices while the pigment and phosphor particles are in aqueous suspension. More particularly, the binding agents are latex polymers with a glass transition temperature of greater than about 20.degree. C. These polymer binders produce tack-free, nonblocking, free flowing, pigmented phosphor powders.
Many methods of binding pigment particles to phosphor particles are described in the literature. These materials are primarily used in cathode ray tubes where the pigment acts as a filter to improve the color purity and contrast of the emitted light. Both organic and inorganic pigment binders are used in the industry. U.S. Pat. No. 4,473,634 describes an inorganic binder system which uses water glass (potassium silicate) as the binder. Organic binder systems such as gelatin, described in U.S. Pat. No. 3,275,466 and various organic polymers, applied as latices, as described in U.S. Pat. No. 4,049,845, have also proved useful to the industry. However, the method as taught in U.S. Pat. No. 4,049,845 utilizes latex polymers that show a glass transition temperature, Tg, well below room temperature. These polymers are therefore rather soft and sticky and result in pigmented phosphor powders which are not free flowing and which aggregate easily. As a result, these powders are rather difficult to screen through a dry seive, which is the normal method used to ensure that the pigmented phosphor powder is deagglomerated. The seiving step is usually the final step in the manufacture of the product. Also, there is a tendency for the pigmented phosphor powder to re-aggregate after screening due to the stickiness or "tackiness" of the polymer binder. U.S. Pat. No. 4,049,845 discusses this problem and methods to reduce tackiness are discussed, including further processing steps such as application of inorganic and organic cross-linking agents. Such steps add to the complexity of the process and may not be effective. Phosphors prepared according to U.S. Pat. No. 4,049,845 undergo so much reagglomeration that it is necessary to wet ball mill these materials in the application slurry prior to their application to the cathode ray tube faceplate. If such a deagglomeration step were not used, the phosphor coating would be grainy and unacceptable. It is desirable to eliminate the wet ball milling step prior to the application of the phosphor to reduce handling which may degrade the phosphor and to reduce expense.
Therefore, a method of coating phosphors with pigment which would eliminate the problems associated with tacky polymer binders, such as poor dry screenability and lumping or agglomeration subsequent to screening would be an advancement in the art. Such improved phosphors would not require milling the phosphor slurry prior to use in the application. Such phosphors are known in the art as "no-mill" or "stir-in" phosphors.